A simple and practical method is described for harnessing the biosynthetic potential of the erythromycin pathway to generate novel molecules of natural product-like complexity. A genetic block has been introduced in the first condensation step of the polyketide biosynthetic pathway that leads to the formation of 6- deoxyerythronolide B (6-dEB), the macrocyclic precursor of erythromycin. Exogenous addition of designed synthetic molecules to small scale cultures of this null mutant results in highly selective multi-milligram production of unnatural polyketides, including aromatic and ring-expanded variants of 6-dEB. Unexpected incorporation patterns are observed, illustrating the catalytic versatility of modular olyketide synthases. Further processing of some of these scaffolds by post-polyketide enzymes of the erythromycin pathway results in the general of novel antibacterials with comparable in vitro potency to their natural counterparts. This method should be applicable to other biologically and medicinally interesting polyketide natural products with minimal, and possibly even no, biosynthetic information at the genetic level.